This paper addresses the problem of simplicity bias (SB) in electrocardiogram (ECG) analysis. SB is a phenomenon in which supervised learning-based ECG models focus on repetitive patterns that are easy to learn, thereby overlooking subtle but clinically important signals. In this study, we first empirically demonstrate the existence of SB in ECG analysis and its impact on diagnostic performance degradation, and find that self-supervised learning (SSL) can alleviate SB. Based on this, we propose a novel SSL-based method that utilizes filters that capture time-frequency features and multi-resolution prototype reconstruction. In addition, we construct a large-scale multicenter ECG dataset containing more than 1.53 million ECG records and demonstrate the superiority of the proposed method through experiments on three subtasks, and we plan to make the code and dataset public.